This invention relates to the display of bistatic passive radar systems. Ordinary (monostatic) radar systems have a transmitter and a receiver located at the same site. The transmitter emits pulses of electromagnetic radiation and the receiver detects reflected radiation from targets illuminated by the transmitter pulse. The range of the target is determined by the time it takes a pulse of a electromagnetic radiation to travel from the transmitter to the target and then by reflection from the target back to the receiver. The transmitter pulses are focused in a narrow beam, and the bearing of the target is determined by the bearing of the transmitter's antenna at the time the reflected pulse is received.
When used in military aircraft or ships, monostatic radar has the disadvantage that the transmitter can be detected at long range (hundreds of miles) by the electromagnetic pulses it emits. This allows the enemy to detect the presence of a ship or aircraft and also to determine its bearing. To get around this disadvantage, bistatic passive radar was developed. Bistatic passive radar does not have a transmitter but rather has two receivers which utilize the radiation emitted by any monostatic radar in its reception area. The transmitter of a monostatic radar system which is being used by a bistatic passive radar system is known as the host transmitter. The bistatic passive radar system locks onto the host transmitter's pulse train, measures the rotational speed of its antenna and its bearing angle, and generates a plan position indicator (PPI) display from this data. Target returns are displayed on the PPI display which has the host transmitter as its center.
In a monostatic radar system, the following relationship exists between the elapsed time for a target return and the target distance from the radar site: EQU .DELTA.tv=2d
Where .DELTA.t equals elapsed time in seconds for the transmitted pulse to reach the target and return, v equals velocity of propagation in feet per second, and d equals distance in feet from the target to the radar site.
This relationship is true for all targets detected by monostatic radar systems. However, for the bistatic passive radar system, this relationship is true only for special targets. The range of most targets on a bistatic passive PPI display are geometrically distorted. For example, signal returns from a target located on either side of a straight line between the host transmitter and the bistatic passive receiver would appear to be closer to this line than it would if the above equation is used. Therefore, there is some geometrical distortion to the PPI display on a bistatic passive radar if no provision is made to compensate for the separation distance between the host transmitter and the bistatic passive receiver.
In the past there have been several attempts to correct this display distortion. One of these used a hybrid digital/analog computer which generated a non-linear range sweep to properly position the target return on the PPI display. This approach was unsatisfactory because the analog circuits are subject to drift.
A full digital display corrector was used in which the target returns were stored in a memory as they were received and later retrieved for displaying at the correct time. However, this system required much hardware and power. The total chip package count was in excess of 100 units. The power requirement was in excess of 50 watts. The principal object of this invention is to provide a digital display corrector for bistatic passive radar displays which is simple in structure and requires only a small amount of power.